Illumination unit and image reading apparatus

ABSTRACT

An illumination unit large in the quantity of light, compact in size and simple in construction without being susceptible to change in ambient temperature is disclosed. In the unit, a cathode 1 for emitting electrons, an anode 3 for accelerating the emitted electrons, fluorescent substances 4G, 4R and 4B for emitting luminescence due to collision of the electrons thereagainst are provided within an air-tightly closed vacuum vessel 20. Further, back electrodes 2G, 2R and 2B for controlling the electron emission area are arranged in the vicinity of the cathode 1.

TECHNICAL FIELD

The present invention relates to an illumination unit and an imagereading apparatus using the same illumination unit, suitable for use inan image scanner, digital copying machine, facsimile, etc. to readimages into a computer, mainly.

BACKGROUND ART

An example of prior art image reading apparatus is disclosed in JapanesePublished Unexamined (Kokai) Patent Application No. 54-81715, forinstance. FIG. 23 is a perspective view showing the prior art imagereading apparatus disclosed in this document, and FIG. 24 is across-sectional view showing the carriage thereof, taken along the lineB--B' in FIG. 23. In FIG. 24, an illumination unit 36 is composed ofthree image illuminating mercury fluorescent lamps of a red mercuryfluorescent lamp 37, a green mercury fluorescent lamp 38, and a bluemercury fluorescent lamp 39, each having a predetermined distribution ofspectral radiant energy and three voltage sources (not shown) forturning on respective lamps. These three red, green and blue mercuryfluorescent lamps 37, 38 and 39 are turned on periodically andindependently in sequence to illuminate an object 28 to be image read.The light reflected by the object 28 is image-formed on a line sensor 34composed of a plurality of light-electricity transducing elements,through a reduction lens 33, in order to read the image by detectingchrominance signals, in sequence.

In the prior art fluorescent lamps, since the evaporation pressure ofthe mercury enclosed inside fluctuates according to ambient temperature,there exists a problem in that the quantity of light fluctuatesaccording to the ambient temperature. In addition, in the case of thefluorescent lamp, the quantity of light obtainable is inevitably limitedand further an appropriate tube diameter of the lamp is as large as 40mm to maximize the luminous efficiency. Therefore, the volume of thethree fluorescent lamps which constitute the color image scanner islarge, and thereby it has been difficult to reduce the size of theillumination unit.

In addition to the above-mentioned problem, the prior art methodinvolves another problem in the case where an object having slopedportions as with the case of an opened book is image read. In moredetail, with reference to FIG. 25, when an object 28 having slopedportions is placed on a table 27 for image reading, the shade of colorswhich are not actually exist (referred to as shear in color image) isproduced, thus resulting in a problem in that the color reproducibilityis deteriorated markedly.

The causes of the above-mentioned problem are described in furtherdetails with reference to FIG. 25. When taking into account a point A atwhich the object 28 is not parallel to the table 27, the object at pointA is illuminated sufficiently by the light emitted from the green andblue mercury fluorescent lamps 38 and 39, but not illuminatedsufficiently by the light emitted from the red mercury fluorescent lamp37. This is because the angle of incidence of light emitted from the redmercury fluorescent lamp 37 upon the object is small at point A.Therefore, in the case where the color of the object at point A iswhite, in spite of the fact that the quantity of light reflected fromthe object at point A must be equal to each other in the three colors ofred, green and blue emitted by the respective mercury fluorescent lampsso that the line sensor 34 can generate the red, green and blue coloroutput signals at the same level, since the red light emitted from thered mercury fluorescent lamp 37 is not sufficient at point A, the redcolor signal output of the line sensor 34 is low, with the result thatthe color at point A is reproduced in a purplish blue color.

In other words, the above-mentioned phenomenon occurs due to the factthat the spread angle a of mercury fluorescent lamp relative to anobject, that is, the angle between the red mercury fluorescent lamp 37and the blue mercury fluorescent lamp 39 obtained when seen from anygiven position on the object to be read is as large as 100 to 120degrees in the case of the prior art illumination unit. Theabove-mentioned problem may be solved by collecting the three mercuryfluorescent lamps at the same position to reduce the spread angle a tothe object. In the case of the prior art illumination unit, however,since the optimum diameter of the fluorescent lamp is as large as 40 mm,it has been difficult to arrange the three fluorescent lamps at the sameposition.

Further, in the prior art illumination unit, since the three lamps areturned on periodically and independently in sequence, it is impossibleto use the same single lamp activating power source, in common for thethree lamps. As a result, when the prior art illumination unit ismounted on the carriage 35 of the image reading apparatus, a large spaceis inevitably required. In addition, since the carriage 35 is moved bythe distance corresponding to the image-reading dimension of the object,there exists a problem in that the size of the image reading apparatusitself is inevitably increased to that extent.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an illumination unitstable in the quantity of illumination light without being susceptibleto change in ambient temperature.

Another object of the present invention is to provide an illuminationunit compact in size and large in light emission capability.

Further, the object of the present invention is to provide a color imagereading apparatus compact in size.

Another object of the present invention is to provide a color imagereading apparatus provided with an illumination unit high in colorreproducibility, without causing any shear in color image reading, evenwhen an image having stepped portions as with the case of an opened bookis image read.

To achieve the above-mentioned object, the present invention provides anillumination unit which comprises within an air-tightly closed vacuumvessel: a cathode for emitting electrons by application of a firstpredetermined voltage; an anode for accelerating the electrons emittedby said cathode by application of a second voltage higher than the firstvoltage; and a fluorescent substance for emitting cathode luminescencelight caused by collision of the accelerated electrons thereagainst.

Further, the present invention provides an illumination unit whichcomprises within an air-tightly closed vacuum vessel: a cathode foremitting electrons by application of a first predetermined voltage; ananode for accelerating the emitted electrons by application of asecond-predetermined voltage higher that the first voltage; a pluralityof fluorescent substances for emitting cathode luminescence light causedby collision of the accelerated electrons thereagainst; and a deflectionelectrode for deflecting a travel direction of the emitted electrons byapplication of a third voltage higher than the first voltage but lowerthan the second voltage, the third voltage being so controlled that theelectrons are directed to any one of a plurality of the fluorescentsubstances.

Further, the present invention provides an image reading apparatusprovided with the above-mentioned illumination unit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a first embodiment of theillumination unit according to the present invention;

FIG. 2 is a cross-sectional view taken along the line A--A' shown inFIG. 1;

FIG. 3 is a diagram showing a circuit for activating the firstembodiment of the illumination unit;

FIG. 4 is a perspective view showing a second embodiment of theillumination unit according to the present invention;

FIG. 5 is a cross-sectional view taken along the line A--A' shown inFIG. 4;

FIG. 6 is a diagram showing a circuit for activating the secondembodiment Of the illumination unit;

FIG. 7 is a perspective view showing a third embodiment of theillumination unit according to the present invention;

FIG. 8 is a cross-sectional view taken along the line A--A' shown inFIG. 7;

FIG. 9 is a diagram showing a circuit for activating the thirdembodiment of the illumination unit;

FIG. 10 is a cross-sectional view showing an essential portion of afourth embodiment of the illumination unit according to the presentinvention;

FIG. 11 is a front view showing a back electrode of metal vapordeposition type used in the fourth embodiment;

FIG. 12 is a cross-sectional view showing an essential portion of afifth embodiment of the illumination unit according to the presentinvention;

FIG. 13 is a perspective view showing a sixth embodiment of theillumination unit according to the present invention;

FIG. 14 is a cross-sectional view taken along the line C--C' shown inFIG. 13;

FIG. 15 is a front view showing a grid 18 shown in FIG. 13;

FIG. 16 is a graphical representation showing the brightnessdistribution on the light generating surface obtained when electrons arecontrolled by use of a grid 19 formed with three openings of the sameopening areas S₁, S₂, and S₃ ;

FIG. 17 is a front view showing a grid 23 of a seventh embodiment of theillumination unit;

FIG. 18 is a perspective view showing a grid 24 of an eighth embodimentof the illumination unit according to the present invention;

FIG. 19 is a diagram showing a circuit for activating the eighthembodiment of the illumination unit, together with a diagrammaticalcross-sectional view taken along the line E--E' shown in FIG. 18;

FIG. 20 is a perspective view showing an embodiment of the image readingapparatus using the illumination unit according to the presentinvention;

FIG. 21 is a cross-section view showing a carriage 26 shown in FIG. 20;

FIG. 22 is a perspective view showing another embodiment of the imagereading apparatus using the illumination unit according to the presentinvention;

FIG. 23 is a perspective view showing a prior art image readingapparatus;

FIG. 24 is a cross-section view showing the carriage 35, taken along theline B--B' shown in FIG. 23; and

FIG. 25 is an illustration for assistance in explaining the case wherean image having sloped portions such as an opened book is read inaccordance with the prior art method.

BEST MODES FOR EMBODYING THE INVENTION

FIG. 1 is a perspective view showing a first embodiment of theillumination unit according to the present invention; and FIG. 2 is across-sectional view taken along the line A--A' shown in FIG. 1. Thedetailed construction of the illumination unit will be describedhereinbelow with reference to FIG. 1.

A cathode 1 is fine wire made of tungsten and having a diameter from 5to 50 μm. The surface of the cathode is covered with an electronemitting substance such as barium oxide (not shown) to increase thethermoelectron emission efficiency. This cathode 1 is formed bystretching a single cathode wire into S-shape with cathode returningpoles 1b disposed on two cathode fixing members 1a. In theabove-mentioned construction, the cathode can be stretched in a singleprocess, fixed by a pair of electrodes, and formed easily even when anarrow light emitting width is needed. Further, the fluorescentsubstances of three different light colors 4R, 4G and 4B are appliedonto the lower surface of an upper plate 5 made of transparent glassinto a rectangular shape, by screen printing method, for instance. Thefluorescent substances are of materials able to generate cathodeluminescence such as Y₂ O₂ S:Eu, Sm for red color, ZnS:Cu, Al for greencolor, and ZnS:Ag, Al for blue color, in practice. A color linear lightsource of three primary colors can be composed by using thesefluorescent substances. The switching speed of the emitted light colorsof this illumination unit can be determined on the basis of theafterglow time of the fluorescent substances. The afterglow time of thecathode luminescence is short; that is, in the case of the greenfluorescent material 4G and the blue fluorescent material 4B, theafterglow time is 0.1 msec or less and in the case of the redfluorescent material 4R, the afterglow time is about 1 msec. The powdersof these fluorescent substances are applied uniformly at a rate of 1 to10 mg/cm², and an anode 3 made of aluminum and having a thickness of 0.1and 0.4 μm is formed on the applied fluorescent substances through theprocess of vapor deposition.

Three back electrodes 2G, 2R and 2B are formed and arranged by bendinginto a U-shape a conductive and non-magnetic material such as brass oraluminum. The gap distance 12 between the respective back electrodes 2G,2R and 2B and the cathode 1 is about 0.1 mm. By reducing the gapdistance, it is possible to decrease the respective voltages applied tothe respective back electrodes 2G, 2R and 2B.

As shown in FIG. 2, a vacuum vessel 20 is formed by an upper plate 5 towhich the three fluorescent substances 4R, 4G and 4B are applied, twoside plates 6a and 6b, a bottom plate 7, and other side plates 6c and 6dshown in FIG. 1, and evacuated down to a vacuum of 10⁻⁵ to 10⁻⁸ Torrthrough an exhaust pipe 6e. Further, the material of the upper plate 5is lead or glass, for instance.

The operation of the illumination unit as described above will bedescribed hereinbelow, with reference to FIG. 3 which shows a circuitfor activating the illumination unit. Here, only the light emissioncontrol of the green fluorescent substance 4G will be describedhereinbelow by way of example.

To the anode 3, a direct current voltage V₁ of 5 to 30 kV, mostpreferably of 10 kV is applied through a lead terminal 3a as shown inFIG. 1. On the other hand, to the cathode 1, a voltage V₂ is appliedthrough a lead terminal 1a shown in FIG. 1. Thermoelectrons generatedfrom the cathode by Joule heat are attracted and accelerated toward theanode 3 by an electric field generated by the anode voltage V₁.

When the green light is emitted, a control signal is applied to aterminal CTL-G from a control means (not shown) to turn on a transistorTr-G. The transistor Tr-G is connected to the green back electrode 2G bya lead terminal 2G-a shown in FIG. 1. Therefore, the electric potentialof the green back electrode 2G is the-same as that of the cathode 1,because both are connected to each other via the lead terminal 1a shownin FIG. 1. The thermoelectrons emitted from the cathode 1 areaccelerated to the anode 3, and further passed through the anode 3 toexcite the fluorescent substance 4G.

Further, the light emission area on the fluorescent substance can beroughly determined by the U-shaped opening width W of the green backelectrode 2G shown in FIG. 2. For instance, when the green backelectrode 2G having a 2 mm wide opening is used, the light emission areaof the green fluorescent substance 4G is also 2 mm in width.

Further, it is possible to make uniform the light emission on thefluorescent surface by providing the back electrode. This is because thedensity of current applied to the fluorescent substance can be averaged.A large effect can be obtained by making the current density uniform,because it is possible to eliminate the local deterioration of thefluorescent substance and further to retain the original performance ofthe substance for many hours. Further, where the light emission isuniform all over the fluorescent substance, it is possible to obtain anillumination unit of uniform light emission.

When the illumination unit is not activated, a control signal is appliedfrom the control means to the terminal CTL-G, so that the transistorTr-G is turned off and therefore a voltage V₃ is applied to the greenback electrode 2G via a resistor R. As a result, the flow of thethermoelectrons from the cathode 1, that is, the anode current isinfluenced. Namely, when an appropriate negative voltage V₃ is selected,the anode current becomes zero, perfectly. In an experiment, thedistance l₁ between the cathode 1 and the anode 3 shown in FIG. 2 was 30mm; the anode voltage V₁, 10 kV; and the green back electrode voltage V₃was -40 V. The shorter a distance l₂ shown in FIG. 2 is, by the lowervoltage will be controlled this back electrode voltage V₃.

The back electrode voltage required to cut off the anode current can beobtained on the basis of the calculation of the field strength inaccordance with the following equation:

    V.sub.3 =-l.sub.2 /l.sub.1 ·V.sub.1

In the same way as described above, it is possible to control the lightemission of the three kinds of fluorescent substances, independently.

FIG. 4 is a perspective view showing a second embodiment of theillumination unit according to the present invention. FIG. 5 is across-sectional view taken along the line A--A' shown in FIG. 4.

In this embodiment, a cathode 8 formed by a single fine wire is sodisposed as to be enclosed by a U-shaped back electrode 9. In the sameway as in the first embodiment, the fluorescent substance having thewidth the same as the opening width W of the back electrode 9 can beexcited to emit light. In more practical, when the distance between theanode 3 and the cathode 8 is l₁ =30 mm; the distance between the cathode1 and the back electrode 9 is l₂ =0.5 mm; the distance between thecathode 1 and the opening of the back electrode 9 is l₃ =9.5 mm; theopening width of the back electrode is W=5 mm; and the thickness of theback electrode is t=0.5 mm, the width at which the fluorescent substanceemits light is roughly 5 mm.

In this embodiment, a grid 10 is disposed between the opening portion ofthe back electrode 9 and the anode 3, and further deflection electrodes11a and 11b are disposed between the grid 10 and the anode 3. The grid10 is formed of a conductive and non-magnetic material such as stainlesssteel, brass, aluminum, etc. and formed into a net-shape or a pluralityof slit-shaped openings so that the electrons can be passedtherethrough.

The operation of this embodiment of the illumination unit will bedescribed hereinbelow. FIG. 6 is a diagram of a circuit for activatingthe illumination unit. When a negative voltage V₁₂ is applied to thegrid 10, the grid 10 can control the flow of thermoelectrons, so that itis possible to switch the light emissions of the red fluorescentsubstance 4R, the green fluorescent substance 4G and the bluefluorescent substance 4B, respectively. In other words, it is possibleto control the light emissions of the fluorescent substances under thecondition that the voltages are kept applied to the cathode 8 and theanode 3.

A voltage V_(3a) higher than that V₂ applied to the cathode 8 is appliedto the back electrode 9. Further, a negative voltage can be applied tothe back electrode 9. When a negative voltage is applied to the backelectrode 9, the emission of the thermoelectrons decrease gradually, andfinally reaches a cut-off condition under which no current flows betweenthe cathode 8 and the anode 3. As described above, it is possible tocontrol the flow of thermoelectrons.

The deflection electrodes 11a and 11b disposed between the grid 10 andthe anode 3 serve to deflect the thermoelectrons emitted from thecathode 8 and accelerated toward the anode 3 in the horizontal directionon the paper. To explain this by a practical example, when the three ofthe fluorescent substances 4R, 4G and 4B are all required to beactivated simultaneously, appropriate positive voltages V₁₀ and V₁₁ areapplied to the deflection electrodes 11a and 11b through switches SW₁and SW₂, respectively. Then the thermoelectrons emitted from the cathode8 are attracted by the positive electric field formed by the deflectionelectrodes 11a and 11b, and diffused beyond the opening width W of theback electrode 9 as shown in FIG. 5 to excite the fluorescentsubstances. In other words, all the three kinds of the fluorescentsubstances can be excited to emit all the colors simultaneously.Accordingly, since three fluorescent substances emit lightsimultaneously, the emitted light is white.

When only the green fluorescent substance 4G is required to be excited,appropriate negative voltages V₄ and V₅ are applied between the groundand the deflection electrodes 11a and 11b through the switches SW₁, SW₂,SW₃, and SW₄, respectively. Then, an electrostatic lens can be formed infront of the cathode 8, so that the thermoelectrons converge in thedirection L1 to excite only the green fluorescent substance 4G. In thesame way, in order to excite only the red fluorescent substance 4R,appropriate negative voltages V₄ and V₅ are applied to the deflectionelectrodes 11a and 11b, respectively to form an electrostatic lens infront of the cathode 8 and thereby to converge the thermoelectrons inthe direction L2. In addition, a positive voltage V₇ is applied to thedeflection electrode 11a through the switch SW₃ and further a negativevoltage V₈ is applied to the deflection electrode 11b through the switchSW₄ in such a way that the voltages V₇ an V₈ are superposed upon thevoltages V₄ and V₅, respectively for electrostatic deflection.Therefore, the converged thermoelectrons are bent toward the directionL2 to excite only the red fluorescent substance 4R.

When only the blue fluorescent substance 4B is required to be excited,in the way opposite to the case with the red fluorescent substance 4R,an appropriate negative voltage V₆ is applied to the deflectionelectrode 11a through the switch SW₃ and an appropriate positive voltageV₉ is applied to the deflection electrode 11b through the switch SW₄ soas to be superposed upon the voltages V₄ and V₅, respectively forelectrostatic deflection. Therefore, the converged thermoelectrons arebent toward the direction L3 to excite only the blue fluorescentsubstance 4B.

As described above, it is possible to control the light emission of thefluorescent substances of three kinds, independently by use of a singlecathode, by controlling the light emission operations of the fluorescentsubstances with the use of the back electrode 9, the grid 10 and thedeflection electrodes 11a and 11b. Further, it is possible to controlthe light emissions of the three fluorescent substances under thecondition that voltages are kept applied to the cathode 8 and the anode3. In addition, in the above-mentioned structure, since the lightemissions of the fluorescent substances can be switched on the order ofnanosecond unit, it is possible to improve the switching speed of thelight emission. Further, in the light emission of the present invention,since the light is emitted by the cathode luminescence light, thequantity of light hardly fluctuates according to change in the ambienttemperature, and additionally the starting characteristics of the lightemission are excellent. Further, since the quantity of light can beadjusted easily, it is possible to construct various illuminationapparatus of a broad range from a large output to a small output.

In the embodiment as described above, although the fluorescentsubstances for high tension application such as zinc sulfide or rareearth element based substance has been used, the same results can beobtained by fluorescent substance for low voltage application such aszinc oxide based substance which can be excited by application of a lowvoltage. Further, when white light is required, these fluorescentsubstances of red, green and blue are mixed and applied under dueconsideration of the brightness balance.

Further, the illumination unit of the present invention can be used as alight source for a liquid crystal television projector by disposing aliquid crystal at the junction surface with the upper plate 5 oftransparent glass. In more detail, it is possible to realize a colordisplay unit by illuminating the back surface of a single monochromaticliquid crystal panel with the illumination unit of the presentinvention, and by exciting the three primary color fluorescentsubstances and switching three primary color liquid crystal panels athigh speed in time series fashion in synchronism with respect to eachother.

Further, it is effective to form a thin film on the inner surface of thevacuum vessel 20 by vapor deposition, so that the inner surface is keptat a constant potential to eliminate the influence of external magneticfield.

FIG. 7 is a perspective view showing a third embodiment of theillumination unit used for an image reading apparatus according to thepresent invention, and FIG. 8 is a cross-sectional view taken along theline A--A' in FIG. 7.

In the drawings, a trapezoidal grid 13 constructed by a single elementis disposed between three cathodes 8 and an anode 3, and fixed between abottom glass plate 7 and both end surfaces of two side glass plates 6aand 6b. In the same way as shown in FIG. 1, the cathode is formed bystretching a single cathode wire into S-shape with cathode returningpoles.

FIG. 9 is a circuit diagram showing the third embodiment of theillumination unit activating circuit. When voltage V₁₅ is applied to thethree cathodes 8, the cathodes 8 are heated by Joule heat and thereforethe electron emission substances applied to the vicinity thereof arealso heated up to 400° to 800° C., into a status where thermoelectronsare emitted easily. Under these conditions, when a positive voltage V₁₃is applied to the grid 13 through a switch SW₅, the thermoelectrons areaccelerated and emitted toward the grid 13 and further somethermoelectrons are accelerated, being passed through openings of thegrid 13, toward the anode 3 by an electric field generated by the anode3 to which a high tension V₁ of 5 to 30 kV, most preferably 10 kV isapplied. Then, the thermoelectrons penetrate through the anode 3 toexcite the fluorescent substances 4G, 4R and 4B, so that visible lightcan be emitted.

Here, only the case of the green fluorescent substance 4G will bedescribed hereinbelow by way of example, with reference to FIG. 9. Underthe condition that a voltage V₁₅ is applied to the cathode 8 and avoltage V₁ is applied to the anode 3, a positive voltage V₁₃ is appliedto the grid 13 via a switch SW₅ ; negative voltages V₁₉ and V₂₁ areapplied to metallic back electrodes 12R and 12B via switches SW₇ and SW₈via, respectively; and a potential the same as that of the cathode 8 ora positive voltage V₁₆ is applied to a metallic back electrode 12G via aswitch SW₆. At this time, since the negative voltage is kept applied tothe metallic back electrodes 12R and 12B, thermoelectrons are notemitted from the cathode 8 corresponding to these electrodes, so thatred light and blue light are not emitted. Accordingly, only the greenfluorescent substance 4G is excited and therefore emits green light.

In the same way, when only the red fluorescent substance 4R is requiredto emit light, negative voltages V₁₇ and V₂₁ are applied to the metallicback electrodes 12G and 12B via switches SW₆ and SW₈ ; and further apotential the same as that of the cathode 8 or a positive voltage V₁₈ isapplied to a metallic back electrode 12R via a switch SW₇. Further, whenthe blue fluorescent substance 4B is required to emit light, negativevoltages V₁₇ and V₁₉ are applied to the metallic back electrodes 12G and12R via switches SW₆ and SW₇ ; and further a potential the same as thatof the cathode 8 or a positive voltage V₂₀ is applied to a metallic backelectrode 12B via a switch SW₈.

Further, the quantity of the electrons emitted from the cathode 8becomes the maximum when the positive voltages V₁₆, V₁₈, and V₂₀ appliedto the back electrodes of the fluorescent substances required to emitlight become a voltage the same as that applied to the cathode 8 (in thesame potential as that of the cathode 8).

Here, the case where the light emission of the fluorescent substances iscontrolled by use of only the back electrodes, without use of the grid13 is taken into account. Under the conditions that the anode voltage is10 kV; the distance between the cathode and the anode is l₁ =21.5 mm;the distance between the back electrode and the cathode is l₂ =0.25 mm;and the voltage applied to the cathode 8 has been adjusted so that theanode current is 50 μA, in order to reduce the anode current down tozero volts, the negative voltage of about -300 V is required to beapplied to the back electrodes 12G, 12R and 12B.

In the present invention, however, since the grid 13 is disposed betweenthe anode 3 and the cathode 8 and further a positive grid voltage V₁₃ of2 to 3 V is applied to the grid 13, it is possible to reduce the backelectrode voltage for controlling the light emission of the fluorescentsubstances to such an extent as low as -50 V. This is because since thegrid 13 is disposed between the cathode 8 and the anode 3, there existsan effect to shield the electric field generated by the high tensionapplied to the anode 3. Therefore, the mission rate of the thermalelectrons from the cathode 8 is dependent upon only the relationship involtage level between the back electrodes 12G, 12R and 12B, withoutbeing subjected to the influence of the voltage of the anode 3.

Further, when no color light is required to be emitted, negativevoltages V₁₇, V₁₉ and V₂₁ are applied to the metallic back electrodes12G, 12R and 12B, respectively, or alternatively a negative voltage V₁₄is applied to the grid 13.

As described above, the electrons emitted from the cathode 8 can becontrolled by use only the back electrodes 12G, 12R and 12B or only thegrid 13. However, it is possible to control the electron emission by alower voltage when controlled by use of both the back electrodes 12G,12R and 12B and the grid 13.

Further, in this embodiment, since the grid 13 is constructed by asingle common element, the supporting structure of the grid 13 can besimplified. Further, since the electrons emitted from the cathode 8cannot reach the anode 3 by passing through portions other than openingsof the grid 13, it is possible to allow only the required fluorescentsubstances to emit required light, without emitting light not required,thus preventing the leakage of the electrons effectively. Therefore, itis possible to control the emission of light of the fluorescentsubstances 4G, 4R and 4B by the metallic back electrodes 12G, 12R and12B and the grid 13, under the condition that voltage is kept applied tothe cathode 8 and the anode 3. As a result, it is possible to emit lightof three primary colors independently from the light emitting sectionsof a single illumination unit.

Further, in the illumination unit of the present invention, since thecathode luminescence light is emitted, the light emission efficiency ishigh. For instance, in the case of the green fluorescent substance ofZnS:Cu, Al, when the anode voltage is V₁ =8 kV; and the current densityof the electrons emitted onto the fluorescent substance is 50 μA/cm² ;the brightness is 47, 420 cd/m². Further, when a large quantity of lightis required, it is possible to obtain the required quantity of light byincreasing the anode voltage and the current density of the electronsemitted onto the fluorescent substance. As a result, it is possible toreduce the light emission area, as compared with the case of theconventional fluorescent lamp, when the same quantity of light isrequired to be emitted.

FIG. 10 is a cross-sectional view showing a fourth embodiment of thepresent invention, and FIG. 11 is a front view showing the metallic backelectrodes of vapor deposition type of the fourth embodiment. A greenback electrode 14G of vapor deposition type, a red back electrode 14R ofvapor deposition type and a blue back electrode 14B of vapor depositiontype are directly formed by selectively depositing a metal such asaluminum on a glass plate 7 into a strip shape by vapor depositiontechnique. Therefore, it is possible to eliminate the manufacturingprocess of fixing the back electrodes between the bottom glass plate 7and the side glass plates 6a and 6b; that is, the back electrodes can beformed easily within the vacuum vessel 20. Further, since the distancebetween the cathode 8 and the back electrodes 14G, 14R and 14B can bemaintained accurately, it is possible to reduce the dispersion of thecontrol voltage applied to the back electrodes 14G, 14R and 14B in orderto excite and not to excite the fluorescent substances.

FIG. 12 is a cross-sectional view showing a fifth embodiment of thepresent invention. In this embodiment, a flat plate grid 14 is formedbetween the cathode 8 and the anode 3, and the grid 14 is fixed betweenthe two divided glass plates 6a and 6b. Therefore, it is possible toeliminate the manufacturing process of bending the grid, and to reducethe material required to form the grid.

FIG. 13 is a perspective view showing a sixth embodiment of theillumination unit of the present invention. Further, FIG. 14 is across-sectional view taken along the line C--C' in FIG. 13, and FIG. 15is a front view showing a grid electrode 18 shown in FIG. 13.

In FIG. 13, the fluorescent substances 15 are applied onto the innersurface of the upper plate 5 being arranged in the x-axis direction inthe order of a green fluorescent substance 15G, a red fluorescentsubstance 15R, and a blue fluorescent substance 15B. In the same way,the back electrodes 16 of the number same as that of the fluorescentsubstances are formed on the side of the cathode 17 opposite from theanode 3 in the order of the green back electrode 16G, the red backelectrode 16R and the blue back electrode 16B being arranged in thex-axis direction. Further, the cathode 17 is stretched one for each pairof the fluorescent substance and the back electrode, in the Y-axisdirection. The grid 18 is formed integrally by etching a plate so as tobe formed with a plurality of opening portions 18a of the number same asthat of the fluorescent substances, as shown in FIG. 15.

Here, when the light emission is electronically controlled by use of thegrid formed with the same opening areas S₁, S₂, and S₃ in the openingportions 18a shown in FIG. 15, the brightness is not uniform over thelight emission surface.

The reason why the irregularity occurs in brightness will be describedin details hereinbelow. FIG. 16 shows the distribution of the brightnesson the light emission surface of the fluorescent substance 15Gelectronically controlled by use of a grid electrode 19 formed with theopenings of the same opening areas S₁, S₂, and S₃. When the cathode 17is heated, although the thermoelectrons 22 are generated, since thevicinities of both end portions P_(S) of the cathode 17 are in contactwith cathode fixing members 21, the generated heat is easily conductedoutside. In other words, the temperature is high at the central portionP_(C) and low at both end portions P_(S), so that there exists adifference in temperature between the central portion P_(C) and both theend portions P_(S). As a result, the quantity of the thermoelectrons 22emitted from the cathode 17 is large at the central portion P_(C) andsmall at both the end portions P_(S). Therefore, under these conditions,when thermoelectrons 22 are irradiated upon the fluorescent substancesby electronically controlling the light emission by use of the grid 19formed with the same opening areas S₁, S₂, and S₃, the brightness L_(C)at the central portion P_(C) is higher than the brightness L_(S) at theend portions P_(S). As a result, the brightness is not uniform, so thatthe central portion of the fluorescent substance deteriorates earlier ascompared with the other portions. In FIG. 16, the brightnessirregularity in the y-axis direction in FIG. 13 has been explained byway of example. However, the same brightness irregularity occurs in thex-axis direction in FIG. 13.

Therefore, in this embodiment, as shown in FIG. 15, the opening area S₁at the end of the opening portion 18a is made larger; and the more theopening areas S₂ and S₃ of the opening portion 18a reach the centralposition of the opening portion 18a, the smaller will be made theopening area of the opening portion 18a. When this grid 18 iselectronically controlled, instead of the grid 19 formed with theopening portions of the same opening areas as shown in FIG. 16, it ispossible to pass a small amount of the thermoelectrons 22 through thegrid at the central portion where the quantity of the thermoelectrons islarge, and to pass a large amount of thermoelectrons 22 through the gridat both the end portions where the quantity of the thermoelectrons issmall. In other words, it is possible to make uniform the quantity ofthe thermoelectrons 22 to be passed through the grid 18 over the grid,by adjusting the opening area of the opening portion 18a of the grid 18,as shown in FIG. 15. As a result, the current density of thethermoelectrons irradiated upon the fluorescent substance can be madeuniform, so that it is possible not only to eliminate the irregularbrightness on the light emission surface both in the x- and y-axisdirections in FIG. 15, but also to prevent the partial deterioration ofthe fluorescent substance.

FIG. 17 is a front view showing a seventh embodiment of a grid 23 of thepresent invention. The opening portion 23a of the grid 23 is formed intosuch a hexagonal bow-tie-shape that the width h₂ at the central portionis smaller than the width h₁ at both the end portions, and further theopening area of the opening portions 23a are formed so as to becomesmaller in the same shape in arrangement toward the central line D--D'of the grid 23. Owing to the structure as described above, it ispossible not only to prevent the irregular brightness in both the x- andy-axis directions in FIG. 15, but also to simplify the manufacturingprocessing of forming the opening portions, as compared with the casewhere a large number of small opening portions as shown in FIG. 15 areto be formed. In addition, there exists another effect such that thequantity of the thermoelectrons blocked by the grid 23 can be reduced,as compared with the case of the grid 18 shown in FIG. 15.

FIG. 18 is a perspective view showing an eighth embodiment of the grid24 of the present invention, and FIG. 19 is circuit diagram foractivating the illumination unit, together with a cross-sectional viewtaken along the line E--E' shown in FIG. 18. In this embodiment, thegrid 24 is formed into a semi-cylindrical shape expanded toward theanode 3.

The case where only the red fluorescent substance 4R is activated istaken into account, by way of example. Under the condition that apositive voltage V₁ is applied to the anode 3; a positive voltage V₁₈ isapplied to the back electrode 14R; and further a positive voltage V₁₃ isapplied to the grid 24, thermoelectrons are emitted toward the grid 24and reach the grid 24. Some of them pass through the opening portion 25of the grid 24, accelerated toward the anode 3 to which a positivevoltage V₁ is applied, and pass through the anode 3 to excite thefluorescent substance 4R, so that red light is emitted. At this time,negative voltages V₁₇ and V₂₁ are applied to the back electrodes 14G and14B in order not to excite the fluorescent substances 4G and 4B.

In an experiment, the width I of the red light emission on thefluorescent substance 4R is about I=2. 4 mm, under the condition that V₁=8 kV; V₁₃ =5 V; V₁₅ =3.3 V; V₁₈ =5 V; V₁₇ =-10 V; V₂₁ =-10 V; thedistance between the fluorescent substance 4R and the grid 24 is J=5.4mm; the distance between the grid 24 and the cathode is K=2.4 mm; thedistance between the cathode 8 and the back electrode 14R is M=1.2 mm;and the radius r of the semi-cylindrical portion of the opening portion25 is r=0.4 mm.

The above-mentioned experiment indicates that since the opening portion25 of the grid 24 is formed into a semi-cylindrical shape, thethermoelectrons are attracted in the x- and y-axis directions in FIG.18, with the result that it is possible to irradiate electrons upon thefluorescent substance 14R uniformly over a width (2.4 mm) wider than theopening width 2r (=0.8 mm) of the opening portion 25 of the grid 24,thus preventing the irregular brightness.

In addition, the above-mentioned method is effective in that it ispossible to prevent the electron leakage to the adjacent light emissionarea and further to secure a sufficient light emission width.

Further, when the shape of the opening area of the opening portion 25 isformed into a bow-tie shape as with the case of the seventh embodiment,it is possible to prevent the irregular brightness in the x- and y-axisdirections, simultaneously.

FIG. 20 is a perspective view showing an embodiment of the image readingapparatus of the present invention, and FIG. 21 is a cross-sectionalview showing a carriage 26 shown in FIG. 20, taken along the line B--B'in FIG. 20. The operation of reading an image will be describedhereinbelow with reference to FIGS. 20 and 21.

Within the carriage 26, an illumination unit 31 the same in constructionas the embodiment of the present invention shown in FIG. 1 is mounted toemit light upon a part of the row L of an object 28 to be image-read andplaced on a glass base 27. The emitted light is reflected from theobject 28 and the reflected scattered light is image-formed onto the CCD34 of the light-electricity transducing element through a mirror 32 anda reduction lens 33. The CCD 34 is of linear image sensor type, andimage data corresponding to only one row can be read by an electriccircuit not shown.

To read color image, the following procedure is taken. Without movingthe carriage 26, first the green fluorescent substance 4G is excited,and the component of the green light reflected from the object 28 isread by the CCD 34; next the red fluorescent substance 4R is excited,and the component of the red light reflected from the object 28 is readby the CCD 34; and further the blue fluorescent substance 4B is excited,and the component of the blue light reflected from the object 28 is readby the CCD 34, so that three primary color components of the object 28can be obtained.

Successively, the carriage on which the illumination unit 31, the mirror32, the reduction lens 33 and the CCD 34 are all fixed is moved by adistance corresponding to the image reading resolving power, in thearrow direction by a driving device 30 through a timing belt 29. Byrepeating the above-mentioned operation, the whole surface of the object28 can be image-read.

In the illumination unit of the present invention, since the colors ofthe emitted light can be switched at high speed, it is possible to readimage of the object 28 by moving the carriage only once; that is, inlinear sequence mode.

Further, in FIG. 21, although the first embodiment of the illuminationunit shown in FIG. 1 is adopted, it is of course possible to mountanother embodiment on the carriage 26.

FIG. 22 is an illustration showing the case where the illumination unitshown in FIG. 7 is mounted on the image reading apparatus. As alreadydescribed, since the light emission area required to secure the samequantity of light can be narrowed, in the case where the light emissionwidth for each color is 1.5 mm; the light emission width for threecolors is 6 mm; and the distance between the light emission plane andthe image is 16 mm, the spread angle a obtained when seeing theillumination unit from a point on the object 28 to be image read isabout 20 degrees, which is about one-sixth of the conventional spreadangle of about 120 degrees as already explained with reference to FIG.25, and thereby it is possible to prevent a shear in color imagereading.

Here, the case is taken into account where the illumination unit shownin FIG. 13 is mounted on the image reading apparatus in the same way aswith the case shown in FIG. 22. In this case, the illumination unit isso arranged that the x-axis direction shown in FIG. 13 corresponds tothe vertical direction with respect to the paper surface in FIG. 22.Under these conditions, the fluorescent substance is arranged in thex-axis direction in FIG. 13, but in the vertical direction with respectto the paper surface in FIG. 21. Therefore, in this case, the spreadangle a of the three color lights from the image is zero in FIG. 22,with the result that there exists a prominent effect such that it ispossible to perfectly eliminate a shear in image reading.

Further, in the illumination unit of the present invention, since only asingle high tension anode power source of large volume is required, itis possible reduce the volume of the carriage when the illumination unitof the present invention is assembled with the image reading apparatustogether with the illumination activating circuit. As a result, it hasbecome possible to reduce the volume of the image reading apparatususing the illumination unit according to the present invention.

In addition, In this embodiment, since the illumination unit is small insize and high in output power, when this illumination unit isincorporated in the image reading apparatus, it is possible to realize ahigh speed and high resolving power image reading apparatus. In thepresent image reading apparatus, in general there is now being adopted aline sensor of integration type which can generate a sensor outputproportional to the integrated value of the quantity of light irradiatedupon an object within an accumulation time. In other words, since thequantity of light sensed by the line sensor increases with increasingquantity of light emitted from the illumination unit to the object to beimage read, in the illumination unit of the present invention, it ispossible to shorten the accumulation time of the line sensor. As aresult, the image reading time can be reduced and therefore the imagereading speed can be increased in the image reading apparatus.Therefore, the illumination unit of the present invention can solve aproblem as to slow image reading speed, which is one of the majorproblems involved in the current image reading apparatus.

According to the present invention, when a voltage is applied to thecathode placed within the air-tightly closed vacuum vessel, electronsare emitted therefrom. The emitted electrons are further accelerated bya high tension applied to the anode and brought into collision againstthe fluorescent substances for emitting cathode luminescence light.

Further, in the present invention, the travel direction of the electronsemitted from the cathode can be deflected by the deflection electrodesso as to be directed to any given fluorescent substances.

Further, in the present invention, the quantity of electrons emittedfrom the cathode can be controlled by the control electrodes. Inaddition, the emission area of the electrons emitted from the cathodecan be controlled by the control electrodes.

As described above, in the present invention, since the illuminationunit emits light on the basis of the light emission principle of cathodeluminescence, it is possible to provide an illumination unit stable inthe quantity of light, without being susceptible to change in theambient temperature. Further, it is possible to provide an illuminationunit compact in size and large in light output.

Further, it is possible to provide a compact illumination unit foremitting three-primary color light by a single cathode so as to becontrollable independently.

Further, in the image reading apparatus using the illumination unitaccording to the present invention, since the illumination unit emitslight on the basis of the principle of cathode luminescence, the lightemission is excellent in starting characteristics and stable in thequantity of light, thus providing a high performance image readingapparatus.

Further, according to the present invention, since the electrons forexciting a plurality of the fluorescent substances are controlled by asingle grid and a plurality of back electrodes within the light emissiontube, it is possible to provide an illumination unit which can emitlight of three-primary colors independently by the light emittingportion of a single illumination unit, and therefore it is possible toprovide an image reading apparatus free from a shear in the color imagereading operation and high in color reproducibility, even in the casewhere objects to be image read have stepped portions such as when a bookis opened.

I claim:
 1. An illumination unit which comprises:a cathode bridged overa bottom of a vacuum vessel, for emitting electrons by application of afirst predetermined voltage; a grid electrode having opening portionsseparated by predetermined distances, the opening portions havingdifferent opening areas in a direction in which said cathode is bridged;an anode for accelerating the electrons emitted by said cathode andhaving passed through the opening portions of the grid electrode byapplication of a second voltage higher than the first voltage; and afluorescent substance for emitting cathode luminescence caused bycollision of the accelerated electrons thereagainst, all the elementsbeing disposed within an air-tightly closed vacuum vessel.
 2. Theillumination unit of claim 1, which further comprises a controlelectrode for controlling a quantity of electrons emitted from saidcathode by application of a predetermined voltage higher than the firstvoltage but lower that the second voltage.
 3. An image reading apparatuscomprising the illumination unit as claimed in claim
 1. 4. Theillumination unit of claim 1, wherein said opening areas become smalleras closer to a center of said cathode in the direction in which saidcathode is bridged.
 5. The illumination unit of claim 1, wherein saidgrid electrode is applied with a predetermined voltage higher than thefirst voltage but lower than the second voltage, thus controlling aquantity of electrons to be accelerated by said anode.